Building a chemical fume hood with a plain steel blower _hood_

I was going through some old papers from my first refinery and came across some sketches I thought some may find useful. This was the first hood I ever made and, all things considered, it wasn't bad. There are a few options I would add today, having had the experience of refining a few ounces under my belt, and maybe we can get some discussion going about what others have experienced and would do to change this design.



Because the blower is basically causing the exhaust by powering an eductor, it only sees room air, not fume laden air so any blower will do. The duct from the blower should be PVC along its entire length but especially from where it enters the hood and goes up into the stack. All elbows should be the biggest sweeps you can find so the air bends around the corner not bangs into a 90 degree wall like it will with a more compact elbow. One alternative is to use 2, 45 degree elbows with a gap equal to the diameter of the duct between them if you can't find an affordable sweep.

One benefit of the blower was the ability to choose where the input air came from. So in summer I used air from the shop to feed the blower but in winter I used outside air. This made heating the shop easier as the only air I had to re-heat was the air sucked out by the draw of the eductor effect.

The window folded up so I could work in the hood and when down I could still stick my hands in and do things. Back then I was a relatively small operation and the hood was capable of handling my production which rarely exceeded I kilo of fine gold a day before I needed to upsize. I controlled the rate I generated NOx by controlling the nitric additions so I did not use a scrubber with this setup. When I got larger I did, and the hoods and scrubber feeds changed but that's another story.

The bottom of the hood was sealed and came up about 4" all around to act as a catch for all spills. (If you say you never have spills then you are one of two things, either a bold faced liar or new at this!)

One of the things I would add to this system would be a waste drainage port. Sounds complicated but it is simply a bulkhead fitting permanently glassed into the bottom to feed into a drum for waste. The translucent drum (so you can see how full it is easily!) is placed under the hood so the legs of the hood must be tall enough. Then a 2" threaded pipe is threaded into the drum bung which is centered beneath the bulkhead so that you can thread the fitting on by looking down through the bulkhead fitting inside the hood and thread in the pipe. The pipe should extend a few inches above the bulkhead and you can cap it with an easily removable cap. The bulkhead should extend above the base of the hood enough to maintain your spill catching capability. (no sense making a 4" deep reservoir and adding a 2" bulkhead) Offsetting the drain port so it is not in the premium front workspace is a good idea.

Now when you need to pour waste into a drum it is no longer a funnel full of stinky acid poured while holding your breath, it is simply uncapping the pipe, inserting the funnel, and pouring inside the vented hood.

When the time comes to empty the drum, the pipe can be removed from inside the hood and the drum slid out for transfer and processing. I quickly realized that if it settled long enough a little more gold always came down so I modified my setup to have an overflow to a second drum by adding a bulkhead overflow a few inches from the top of the drum under the hood and feeding a shorter drum alongside it. That gave me a long retention time for settling and a yearly draining of the tank to recover the settled values on the bottom. I always used Viton gaskets for the bulkhead fittings and never had a leak with spent aqua regia waste.

I also added lights by cutting a round 10" hole in the top of the hood (one on each side of the exit duct) and glassing in a 10" pyrex glass pie plate. Then a "chicken brooding lamp" can be fitted with a bright bulb and placed on top of the hood, out of the fumes so it will not be eaten up, and you have plenty of light to work with. (The older i get the more I appreciate good light!)

When I first joined the forum I posted about building a hood and I think this information can add on to that post to help out a few members looking to make a lab for themselves.

Any other suggestions, or add ons to make this work better are welcome.

Oh now you tell me! I love reading stories from you gentlemen. Having gotten to where i am now you have to learn those kind of things through trial and error. And boy is there a lot of error! The simplicity of the waste drainage with two settling takes is something that i have had to adapt into my system because of chlorides and fine gold. Chasing them around the system can be a real pita in time and effort. I've learned to use vacuum transfer in almost everything i do now. Makes my job and the safety issue so much easier. I've also had to abandon my once productive fume hood and lab in place of a new building and larger reaction vessels with fume scrubbers like you spoke of. Some days the old lab just seemed like a simpler time.

I've seen a lot of different designs of eductor based fume hoods on the forum and I think that almost every one had the same basic design flaw, the eductor was placed close to the hood. In some cases the long piping added so much drag that the hood got a positive pressure and failed.
If the eductor is placed closer to the exhaust with only a straight pipe and no bends the extra air from the fan will create minimal drag in the system and create a vacuum. In this way the hood will always get a suction and even with a lot of bends of the pipes it will suck out the fumes. The lower air speed in the piping to the hood will also minimize the drag and increase the efficiency.

For comparison I think (i.e. don't know) that virtually every suction dredge have the eductor close to the surface and not at the end of the suction tube.

Göran

There are two types of suction dredge designs, one the eductor is at the nozzle under water, this design uses two hoses underwater, and the other design on the eductor is above water at the sluice box.

A twelve-inch eductor flue here in Minnesota with our prairie winds & weird turbulence....

Would that be inviting stalling or back drafts as building shading pulled negative air pressure on the structure etc.?

Whats the best way to add protection layers from possible back drafts with that type system?

And how about with direct fan/impeller driven system? Thanks :roll:

I can agree that my comparison to a dredge could be a bit lacking... but I checked the last episode of "Bering Sea gold" to look at their suction hoses and none of them had the eductor at the nozzle. :mrgreen: To transport solid objects there is a benefit of higher flow rate so that gives an edge to the eductor in the suction nose.

But I still stand by my statement that placing the eductor too close to the fume hood is bad. You create over pressure in the pipes, any leakage is pressed back into the room. Too much drag can create an overpressure killing the suction totally.

If the eductor is placed close to the exhaust it will create suction in the pipe and any leakage will only lower the suction in the hood, not push the fumes back into the room. Lower flow (no air from the eductor) in the pipes means lower drag and in the end result in more suction in the fume hood.

Göran

I would try a chimney cowl, I have seen them made in metal but you would have to make one in plastic. They have some that use a fin to point the exhaust stack into a direction where the wind can enhance suction.

Goran,

The way an eductor works is to cause a low pressure area below where the eductor tube lets out in the larger pipe. Then air in the hood is drawn up into the area of low pressure and blown out by the exhaust from the fan. They are not as efficient as a blower which has an impeller in the air path but they are not as costly either. If you listen to the concerns of members on the forum it is the cost of a corrosion resistant blower. This design is to satisfy that concern. There are always trade-off's in life, refining is no exception.

Part of most educators is a venturi in the piping, I do not see that in the simple drawing above, just after the point where the fan outlet pipe (fan orifice nozzle output), in the drawing above the exhaust pipe should increase in size to create a venturi effect.

https://www.google.com/#q=venturi
https://www.google.com/search?q=venturi+exhaust+fan&source=lnms&tbm=isch&sa=X&ei=EePnUsWfOpfDoATm9IGgCg&ved=0CAgQ_AUoAg&biw=1024&bih=618

Yes a professionally made venturi does choke down in diameter where the force of the blower exits but this works on the same principle it just costs less to make. I used one of these for years even after I had better professionally made systems this hood was in use. The one thing that is important is to have make up air, this will not pull air out of a negatively pressurized building, that is why in winter when the place was sealed up, I used outside air to feed the fan.

I think that the design that 4metals has posted here is a good framework that anyone can build upon or adapt to their individual requirements.

Nothing is perfect, and certainly everyone has differing needs. Which is why I made the comments in my first sentence. He stood up and offered something for the benefit of the community, but if anybody has built a better working model and is prepared to post it with diagrams then I'm all ears.

Here are some pics of the fume hood I just finished building - it's basically on the same design as 4metals drawing in his first post

The pic where I am hold the black shop vac pipe up at the elbow is showing about how far the 2" PVC pipe goes beyond the elbow inside the 4" pipe so that the air flow though the 2" pipe cause the air to draw though the hood & not just pass over the elbow (it doesn't actually go in quit that far)

The blower fan is one from a propane hot water heater (for vent the CO2) I also have a 120 volt computer fan blowing on the main blower motor to help keep it running cool being as how it runs 24/7

The 4" intake on the blower goes though the wall to the out side of the lab so it doesn't suck the heat out of my lab (the fume hood pulls enough as is)

Kurt

I don't know about most folks here, but venting fumes into the atmosphere just seems like a bad thing to do. I think the Code Enforcement Officer would agree with me. A Scrubber would keep the Authorities in a much better mood.

Good point about the scrubber but it doesn't seem any worse than just processing outside. Quantity of fumes and how close your neighbors or your nice vehicle is would make a difference. Better than a box fan in a window anyway.

Irons

Actually you may be surprised at how much you can vent before they (EPA/DNR - fed/state) becomes concerned

This last year when I was working on a joint venture with a local scrap yard we had to look into it because we had to be legit with the town/state & fed regulations

Based on our expected discharge (which would be fair greater then an hobby operation) we were hardly even on the radar - no scrubber needed (we had one anyway) they more or less just wanted us to have our permit in place & know what we were doing - the permit was dirt cheep - we fell under "very small" producer

Can't remember numbers will see if I can get them

Now that I am back home - I am working out of a lab set up out in the old barn (close to a foot ball field away from the house) my nearest neighbor is close to 1/2 mile away --- the "little" bit I do as a hobby is not an issue --- if I had neighbors living just on the other side of my yard fence on 3 or 4 sides I would consider it an issue

Kurt

Small scrubbers for a heated reaction can be simple to set up and use, they can be beneficial to your local environment (or property) as well as to all of our environment including neighbors, the collection of fumes many time can result in useful acids to reuse, with the price and trouble to get nitric acid, collecting these gases and reusing them can keep a little money in your pocket.

It is true on an industrial scale you are allowed to release a certain amount of environmental pollution as long as you do not release more in the time allowed, and as hobbies we are exempt from some rules that the industry standards have to follow because we are small scale.

BUT

Just try to explain that to the officials who come knocking on your door. Wondering what those toxic and air polluting fume are coming from your shed, explain to them why they should not condemn your property or make it a Superfund clean up because there meters show you are emitting many of the fumes that they condemn properties for (like they do in illegal drug manufacturing processes), try to explain why you are storing toxic sludge on your property, and how you understand how to dispose of it properly...

Try explaining to someone who has no clue, but knows he has the authority to do something about what he perceives is a danger to others and the environment, and who has no clue as to what you are doing or the chemistry behind it, not understanding, he will not think you know what you are doing...

Scrubbing fumes make sense, as well as fume hoods.

I totally agree with Irons and his wisdom, and common sense here:

Venting fumes into the atmosphere is a bad thing to do. I think the Code Enforcement Officer would agree with me. A Scrubber would keep the Authorities in a much better mood.

butcher said:

Small scrubbers for a heated reaction can be simple to set up and use, they can be beneficial to your local environment (or property) as well as to all of our environment including neighbors, the collection of fumes many time can result in useful acids to reuse, with the price and trouble to get nitric acid, collecting these gases and reusing them can keep a little money in your pocket.

Click to expand...

Yes & I agree - I do not use my hood as a place to do primary reaction (heavy fume production) with the hood as the means of evacuating the fumes --- the hood is for working when prepping to do reactions & working with the solutions after reaction & heavy fuming is done

I believe I posted about this being the purpose of the hood at least one time in the past (maybe more) here on the forum

Here is a pic of my scrubber used for heavy fuming reactions & it works with buckets as well as with the flasks

I comments to Irons was simply to point out that when working with the scrap yard we had to be in compliance with the town/state & fed therefore we had to look into it & it is surprising the amount you can discharge

Kurt

I agree with Irons as well but in a lot of states the EPA guidelines uses a "de minimus" status to determine how much NOx you generate. If you use less than 10 pounds of nitric in a day you are under the bar and you can operate with direct exhaust, HOWEVER, commercially you have to apply to be granted the de minimus status.

But this hood is intended to provide a reasonable exhaust ventilation to keep the air clean for the operator, I would still scrub the concentrated NOx fumes through a scrubber and run the air from the hood exhaust as a direct vent.

Correct - which is why I posted the pic of my scrubber used for dealing with the concentrated fumes - the fume hood is a work station - not a reaction vessel

Kurt

I agree, Kurt. A lot of folks use a Fume Hood as a location to do all of their work, when it should be primarily to catch the stray fumes when working with reagents and venting glassware while changing-out reaction flasks and such. The best of both worlds is a fume hood with an outlet to a scrubber.
I don't have any choice, my neighbors are just looking for an excuse to complain. The Code Enforcement officer has already paid me a few visits as a result, so I have to go the extra mile to keep him happy. Luckily, my property is zoned Industrial, so he's restricted on how much he can do.

Maybe these pics explain it better then my words

Also please keep in mind that this is a work in progress - I am still working on re-setting up at home after backing out on the joint venture at the scrap yard - what I am showing is no where near completion - I work on it continually while at the same time working at making a living (the bills do need to be paid)

At the scrap yard both our reaction vessels & fume hoods were set up with scrubbers - we had to large commercial hoods with 12 inch discharge pipe (run through a scrubber)

Kurt